1. Field of the Invention
The present invention relates to an electroacoustic transducer having an air chamber in back of a diaphragm, the electroacoustic transducer actively canceling a sound pressure generated in the air chamber such that the air chamber functions as a large-volume air chamber even if the air chamber has a small volume, and thus enhancing bass response.
2. Related Background Art
Electroacoustic transducers, for example, unidirectional dynamic microphones, omnidirectional dynamic microphones, headphones, and speakers may each have an air chamber to prevent sound waves from entering from the outside. Such an electroacoustic transducer has a diaphragm that vibrates in response to sound waves or generates sound waves as being driven by audio signals. The air chamber is provided in back of the diaphragm. The air chamber functions as an acoustic capacitance. Specifically, a large air chamber functions as a lowly resilient spring, while a small air chamber as a highly resilient spring. Thus, in the case where an acoustic capacitance having a small stiffness is required, namely, the diaphragm can be moved easily, a large-volume air chamber is needed.
The air chamber is explained in more detail in the case of an omnidirectional or unidirectional dynamic microphone as an example herein. In the omnidirectional or unidirectional dynamic microphone, an acoustic resistance and an air chamber should be provided in a rear portion or in back of a diaphragm in order to obtain omnidirectional components. The stiffness of the air chamber is dominant in a low frequency range. If the air chamber has a small volume, the stiffness is high and directional frequency response is low. Thus, the volume of the air chamber must be increased to reduce the stiffness.
In a hand-held wireless microphone, a transmitter circuit and a power battery should be housed in a grip, and thus a large air chamber cannot be provided like a wired microphone. Accordingly, the air chamber in the rear portion of the diaphragm is limited in volume and omnidirectional components should be obtained in a small air chamber. This results in poor directional frequency response and sound quality in bass sound. Specifically, if a small air chamber responds to bass sound to vibrate a diaphragm, a large back pressure is applied to the diaphragm. The diaphragm is then difficult to vibrate, thus increasing the lowest responding frequency level and reducing the bass output level.
The inventor of the present invention invented and filed a patent application of a dynamic microphone reducing an acoustic impedance in a back air chamber in an equivalent manner to allow pickup of bass sound even in a small-volume back air chamber (refer to Japanese Unexamined Patent Application Publication No. 2009-232176). In the invention disclosed in Japanese Unexamined Patent Application Publication No. 2009-232176, the back air chamber is provided in back of a diaphragm of a main microphone unit and a sub-microphone unit is disposed in front of the main microphone unit in a casing that supports the main microphone unit. Audio signals (voltage signals) output from the sub-microphone unit drive a membrane composed of a piezoelectric element in the back air chamber, thus reducing the acoustic impedance in the back air chamber in an equivalent manner.
According to the invention disclosed in Japanese Unexamined Patent Application Publication No. 2009-232176, sound waves from a sound source directed to the sub-microphone unit disposed in front of the main microphone unit are converted into audio signals in the sub-microphone unit. The audio signals drive the membrane composed of the piezoelectric element in the back air chamber. The output signals from the sub-microphone unit disposed in front of the diaphragm of the main microphone unit feedforward-controls the membrane composed of the piezoelectric element. In response to the sound waves from the sound source reaching the sub-microphone unit, a pressure change in the back air chamber is estimated and the membrane is driven based on the estimation. Thus, the membrane cannot be driven properly in accordance with the pressure change in the back air chamber. A further improvement is required for acoustic impedance control in the back air chamber at high accuracy.